1. Field of the Invention
The present invention relates in general to the field of lighting and signal processing, and more specifically to a system and method of time division light output sensing and adjusting the brightness of different spectra of light emitted from light emitting diodes.
2. Description of the Related Art
Light emitting diodes (LEDs) are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives, such as the reduction of mercury. LEDs are a type of semiconductor devices and are driven by direct current. The brightness (i.e. luminous intensity) of the LED approximately varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the LEDs or by reducing the average current through duty cycle modulation.
that is noticeable by a human. Additionally, the brightness of an LED can vary over time due to factors such as age.
FIG. 1 depicts a lamp 100, and lamp 100 includes a housing 101 to enclose components of lamp 100. Lamp 100 also includes a narrow-band light sensor 102 and a controller 104 to adjust power to LED 106 in response to changes in the light output of LED 106. A “narrow-band” light sensor senses light in a narrow spectral band. For example, a narrow-band red light sensor senses red light but does not sense any other color light. In addition to LED 106, lamp 100 also includes LED 108. LED 106 and LED 108 have different spectrum. Thus, the “spectrum” of an LED refers to the wavelength or wavelengths of light emitted by the LED. Wavelengths of light determine the color of the light. Thus, the spectrum of an LED refers to the color of light emitted by the LED. For example, in one embodiment, a blue-green spectrum LED 106 emits blue-green light, and a red spectrum LED 108 emits red light. Lamp 100 receives an alternating current (AC) voltage VAC—SUPPLY from supply voltage source 110 through input terminals 112 and 113. The voltage source 110 is, for example, a public utility, and the AC supply voltage VAC—SUPPLY is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. Power control system 116 includes lamp drivers 114 and 115 that provide respective drive currents iLED1 and iLED2 to LEDs 106 and 108. Drive currents iLED1 and iLED2 are direct currents (DC). Varying the value of DC currents iLED1 and iLED2 varies the brightness of respective LEDs 106 and 108.
Controller 104 controls lamp drivers 114 and 115 to control the respective values of drive currents iLED1 and iLED2. Lamp drivers 114 and 115 are switching power converters. Controller 104 provides a pulse width modulated switch control signal CS00 to lamp driver 114 to control a switch (not shown) of lamp driver 114, and controller 104 provides a pulse width modulated switch control signal CS01 to lamp driver 115 to control a switch (not shown) of lamp driver 115. The values of drive currents iLED1 and iLED2 are proportional to the pulse width and duty cycle of respective control signals CS00 and CS01.
Light sensor 102 is a limited band light sensor that senses the brightness of LED 106 but is insensitive to light emitted from LED 108. The light 118 emitted by LEDs 106 and 108 reflects off the interior surface of housing 101 and propagates through diffuser 120 to generate broad spectrum light 122. Some light from LEDs 106 and 108 is reflected and/or directly transmitted to light sensor 102. Light sensor 102 senses the brightness of blue-green light from LED 106 and sends a signal SEN0 to controller 104 that indicates the brightness of light emitted from LED 106. Controller 104 increases the drive current iLED1 if the brightness of LED 106 light is too low relative to a predetermined target brightness value and decreases the drive current iLED1 if the brightness of LED 106 light is too high relative to a predetermined target brightness value. The predetermined target brightness value is a matter of design choice.
Changes in brightness of an LED over time sometimes relate to the amount of power used by the LED over time. In at least one embodiment, the power that an LED uses over time is directly proportional to changes in brightness of the LED over time. Thus, the brightness of an LED that uses more power will likely change over time prior to any changes in brightness of a similar quality LED that uses less power. For example, LED 108 receives only a small percentage, such as 5%, of the total power provided to LEDs 106 and 108. As a result, the brightness of LED 108 is relatively unaffected over time. LED 106 receives 95% of the power, and, thus, the brightness of LED 106 will most likely change over time. Additionally, the power of the red component of light 122 is relatively small. Since the brightness of LED 108 is assumed to be approximately constant over the life of lighting system 100, no feedback is provided to controller 104 to adjust the brightness of LED 108. Thus, lighting system 100 avoids the cost of an additional light sensor, feedback circuitry, and controller complexity to sense and adjust the red light of LED 108.
FIG. 2 depicts a lighting system 200. Lighting system 200 includes an ambient light sensor 202 to facilitate light harvesting. Light harvesting involves supplementing artificial light 204 with natural light 206 and correlating adjustments in the artificial light with variations in the natural light. In at least one embodiment, “natural light” refers to light not generated artificially, i.e. by lamps, etc. In at least one embodiment, “natural light” refers to sunlight and reflected sun light. The physical location of ambient light sensor 202 is a matter of design choice. In at least one embodiment, ambient light sensor 202 is physically attached to the exterior of lamp housing 208. Location of ambient light sensor 202 on the exterior of lamp housing 208 assists in minimizing the contribution of artificial light 204 to the ambient light 206 received by light sensor 202.
Power control system 211 includes controller 210 to control power provided to light source 214 and, thus, control the brightness of artificial light 204 generated by light source 214. Controller 210 generates control signal CS1 and provides control signal CS1 to lamp driver 212 to control power delivered by lamp driver 212 to light source 214. The particular configuration of lamp driver 212 is a matter of design choice and, in part, depends upon the configuration of light source 214. Light source 214 can be any type of light source, such as an incandescent, fluorescent, or LED based source. Lamp driver 212 provides power to light source 214 in accordance with control signal CS1. Ambient light sensor 202 generates sense signal SEN1. Sense signal SEN1 indicates the brightness of ambient light. Controller 210 causes lamp driver 212 to increase or decrease the brightness of artificial light 204 if the ambient light is respectively too low or too high.
Referring to FIGS. 1 and 2, lighting system 100 includes LEDs 106 and 108 with different spectra. Light source 214 can also include individual light sources, such as LEDs, with different spectra. Although lighting system 100 distinguishes between light sources having different spectra, lighting system 100 has a one-to-one correspondence between light sensors and light source spectrum, i.e. for a light source emitting a light at a particular color, the light sensor senses only light having that particular color. Lighting system 100 saves cost by not sensing light from LED 108 and, thus, avoids adding another light sensor. Lighting system 100 does not use a single, broad spectrum light sensor to sense light from both LED 106 and LED 108 because the broad spectrum light sensor cannot distinguish between the brightness of light from LED 106 and LED 108. Accordingly, controller 104 would not be able to detect if the brightness of LED 106 and/or LED 108 had changed over time. Thus, lighting system 100 exchanges accuracy and control of the brightness of LED 108 for lower cost. Lighting system 200 does not distinguish between light sources of different spectra and, thus, does not customize adjustments to the brightness of light sources based on the spectra of the light sources.